The present invention relates to a transducer for extra-uterine monitoring of fetal heart rate during pregnancy and labor, with a transducer casing the lower portion of which carries a plurality of piezo-electric crystals arranged to emit and/or receive ultrasound waves with wide-beam characteristics.
In gynecology and obstetrics, two medical parameters are important to assess the condition of the fetus. These two parameters are the fetal beat-to-beat heart rate--e.g. measured via an ultrasound doppler signal--and uterus (or labor) activity. Simultaneous assessment of the fetal heart rate (FHR) and uterus activity (Toco) allows an exact determination of the fetal condition. Monitors measuring and recording both parameters are called cardiotocographs (CTG monitors).
Under second stage labor and birth, the fetal heart rate may be obtained directly by a fetal scalp electrode. Uterus activity may be measured by an intrauterine pressure transducer. Although both measuring principles lead to excellent signals, they require intravaginal measurement and are therefore not applicable for checks during pregnancy.
For pre-birth applications, the fetal heart rate may therefore be obtained by an ultrasound (US) transducer which is to be placed externally on the pregnant woman's abdomen (there are also further methods to obtain the fetal heart rate externally like measuring the heart sound, but the ultrasound method is the most common one). The ultrasound signal is received by piezo-electric crystals and appropriately filtered. As the heart rate signal is contained in the--very noisy--received ultrasound doppler signal, the ultrasound transducer must be placed directly over the fetal heart, i.e. on the lower left part of the abdomen.
Uterus activity is obtained by means of a tocodynamometer (toco transducer) which is to be placed externally on the fundus uteri, i.e. approximately centrally on the abdomen. That transducer is a tension measuring device, most commonly one or more resistive wire strain(s). The theory of what is really measured by these transducers is complex as uterus "hardness" as well as uterus deformation (and sometimes also respiration) influence the tension, but still it is possible to obtain clinically relevant results, i.e. to discover contraction frequency and shape.
For clinical checks during pregnancy and in the first stage of birth, both external (US and Toco) transducers have to be placed on the abdomen, but at different places as described above. Each of these transducers must be fixed with a rubber belt. This procedure is time and material consuming as well as unpleasant for the pregnant woman.
One has therefore just tried to combine transducers. A very early cardiotocograph had a transducer which measured the fetal heart rate via the heart sound by means of a moving coil as well as uterus activity by means of a movable bolt the position of which was sensed by a differential transformer. This combination transducer was very weighty (approximately 1.5 kilograms), difficult to apply and uncomfortable for the woman. Furthermore, the fetal heart rate signal was not stable as measuring the heart sound did not yield a reliable signal (in particular as the transducer could not be placed in its optimum position--it had to be placed in a medium position between the optimum points for heart rate and uterus activity). Some women additionally complained about hematoma caused by the movable bolt.
Therefore, one returned to separate transducers for fetal heart rate and uterus activity as described above.
It is a major objective of the present invention to provide a transducer which combines both fetal heart rate and uterus activity measuring means in one housing but still does not comprise the disadvantages of the earlier combination transducer.
According to one aspect of the invention, a transducer for extra-uterine monitoring of fetal heart rate during pregnancy and labor, with a transducer casing the lower portion of which carries a plurality of piezo-electric crystals arranged to emit and/or receive ultrasound waves with wide-beam characteristics comprises additionally tension measuring means supported by said transducer casing for the purpose of measuring labor activity, said tension-measuring means being connected with a movable base plate; furthermore, said base plate carries at least one piezo-electric crystal.
In former ultrasound transducers, the ultrasound beam had a relatively small diameter (approx. 3 centimeters) in order to obtain a meaningful heart rate signal not covered by other noise contained in the ultrasound signal. These ultrasound transducers had to be placed exactly over the fetal heart with the ultrasound beam hitting the heart.
Since several years, wide-beam ultrasound transducers are available. These transducers emit an ultrasound beam with diameter of 4 cm or more (typically 6 cm) and are therefore easier to apply. Still the fetal heart rate can be obtained reliably as the received ultrasound signal is filtered by an auto-correlation filter.